1. Field of the Invention
The present invention relates to a moving image processing apparatus for generating a frame image from a field image in a moving image, a control method thereof, and a program.
2. Description of the Related Art
To transmit and store moving image data, conventionally, an interlaced image (field image) with 60 fields per second is widely used. For transmission of digital broadcasting or digital data storage of DVDs, an interlaced image is compressed using moving image encoding based on MPEG-2. Even shooting by a video camera is generally done using an interlaced image with 60 fields per second.
On the other hand, movie shooting and screening commonly use an image of 24 frames per sec. Hence, recording on a DVD is done normally by converting a progressive image (frame image) having 24 frames per sec into an interlaced image (field image) having 60 fields per sec.
This conversion is called 2-3 pull-down processing in general. FIG. 9A shows frame images A to D at a rate of 24 frames per sec. FIG. 9B shows an interlaced image including 60 fields per sec. The 2-3 pull-down processing divides the continuous frame images A to D into 2-3-2-3 field images A1, A2, B1, B2, B3, C1, C2, D1, D2, and D3. Odd- and even-numbered field images are converted into the field images of an interlaced image in an alternating sequence.
A display for displaying these broadcasting or stored images uses a CRT from long ago. The CRT directly outputs and displays an input interlaced image. However, recent development of new display devices represented by LCDs and plasma displays is popularizing apparatuses for displaying a progressive image converted from an interlaced image.
To convert an interlaced image (field image) into a progressive image (frame image), a current field image and an immediately preceding field image are simply combined to generate a frame image (e.g., Japanese Patent Laid-Open No. 11-88845).
FIG. 9C shows an example of frame image generation using this method. A1/A2 indicates a frame image generated by combining the field images A1 and A2. The field images A1 and A2 are generated from a frame image of the same time. Hence, they can theoretically completely reproduce the original frame image. In this method, however, the next frame image A2/B1 is generated from field images of different times and is therefore visually unnatural.
If it is known that the frame images are initially generated at a rate of 24 frames per sec, and the field images undergo the 2-3 pull-down processing, the initial frame images can be reconstructed by executing processing inverse to the 2-3 pull-down processing. When two frames and three frames of these frame images are reproduced in an alternating sequence, no frame image is generated by combining field images of different times.
FIG. 9D shows the inverse 2-3 pull-down processing. The field image A1 is combined with the immediately succeeding field image A2 to generate the frame image A1/A2. The field image A2 is combined with the immediately preceding field image A1 to generate the frame image A1/A2. The field image B1 is combined with the immediately succeeding field image B2 to generate the frame image B1/B2. The field image B2 is combined with the immediately preceding field image B1 to generate the frame image B1/B2. The field image B3 is combined with the immediately preceding field image B2 to generate the frame image B2/B3.
That is, to generate a frame image by combining field images of the same time, a target field image is combined with an immediately preceding or immediately succeeding field image. Combination with an immediately succeeding field image is represented by “2”. Combination with an immediately preceding field image is represented by “1”. The field image combination pattern is 2-1-2-1-1. Generated frame images have the repetitive pattern of the combination pattern.
Japanese Patent Laid-Open No. 7-107375 discloses an example of a repetitive pattern detection method. The technique of Japanese Patent Laid-Open No. 7-107375 returns an interlaced image at a rate of 60 fields per sec to a progressive image at a rate of 24 frames per sec. The repetitive pattern detection technique is common to that in processing of generating a progressive image at a rate of 60 frames per sec. This method detects the difference between field images to determine whether they are generated from a frame image of the same time. The repetitive pattern of the sets of field images of the same time is detected based on the determination result. Frame images can be generated and displayed in a correct time sequence without any problem as long as the repetitive pattern is detected.
However, an image generated by the 2-3 pull-down processing does not always take the 2-1-2-1-1 repetitive pattern described above. The repetitive pattern may be disordered if, for example, field images that have undergone the 2-3 pull-down processing are edited at an arbitrary point. FIG. 9E shows an example in which the scene after the field image B2 in FIG. 9B is cut, and a scene from a field image M1 is connected.
In this case, an ideal repetitive pattern from the field image A1 is 2-1-2-1-2-1-2-1-1. Japanese Patent Laid-Open No. 7-107375 employs a method of correcting such a phase shift in the repetitive pattern by externally giving the information of the editing point, but mentions no specific resolution to how to detect the editing point.
FIG. 9F shows an example in which the scene after the field image B2 in FIG. 9B is cut, and a scene from a field image L3 is connected. In this case, there is no ideal field image combination for the field image L3. The field image L3 degrades the image quality when combined with either the field image B2 or the field image M1.